1. Field of the Invention
The present invention relates to a cordless telephone comprising base and handset units, and more particularly to improvements of the functions of a cordless telephone device.
2. Prior Art
FIG. 1 shows a circuit block diagram of a conventional cordless telephone CT, which is a PBX terminal of a PBX (private telecommunication exchange). The cordless telephone is connected to the PBX via PBX lines. In FIG. 1, reference numerals 1 and 2 respectively denote base and handset units.
The base unit 1 of the cordless telephone includes an antenna 1.sub.1, a duplexer 1.sub.2, a transmitter 1.sub.3, a receiver 1.sub.4, a control circuit 1.sub.5 and a data transmission-reception circuit 1.sub.6, while the handset unit 2 includes an antenna 2.sub.1, a duplexer 2.sub.2, a transmitter 2.sub.3, a receiver 2.sub.4, a control circuit 2.sub.5, a codec 2.sub.6, a keyboard 2.sub.7, a display (LCD) 2.sub.8, a microphone 2.sub.9 and a speaker 2.sub.10. Transmission and reception of signals between the base and handset units 1 and 2 are carried out via the antennas 1.sub.1 and 2.sub.1.
In a conventional digital telephone system, a PBX terminal is controlled to operate synchronously with a PBX by a synchronization signal provided from the PBX. In a case where a PBX terminal is a cordless telephone as shown in FIG. 1, the digital telephone system requires two synchronization systems. One of the synchronization systems is for synchronizing the PBX with the base unit 1, and the other is for synchronizing the base unit 1 with the handset 2. In addition to this requirement, the synchronization systems need to be synchronized with each other.
Accordingly, in the conventional digital telephone system, the base unit 1 regenerates a synchronization signal on the bases of that received from the PBX using a digital PLL circuit, and sends the regenerated synchronization signal to the handset unit 2 at an interval of, for instance, ten minutes to thereby allow the two synchronization systems to be synchronized with each other for operation.
In order to synchronously control two synchronization systems as described above, however, there are required digital PLL circuits which are particularly precise and thus complicated. In addition thereto, in the digital telephone system, a synchronization clock used in the first synchronization system for synchronizing the PBX and the cordless telephone device varies depending on different PBXS, and this results in a very complicated configuration for synchronization between the first and second synchronization systems.
Each of the base and handset units 1 and 2 shown in FIG. 1 incorporates a ROM storing digital threshold values for received signal strength (RSSI value) in the control circuit 1.sub.5 or 2.sub.5. The stored threshold values are fixed values determined in accordance with detection property of a standard IF circuit which detects a reception signal strength. Such IF circuits are included in RF stages of the receiver 1.sub.4 and 2.sub.4. The detected reception signal strength in a form of an analog voltage signal is converted to a digital signal at the control circuit 1.sub.5 or 2.sub.5, and then is compared with the threshold values stored in the ROM. When the reception signal strength exceeds a certain threshold value, the control circuit 1.sub.5 or 2.sub.5 controls to decrease a transmission power of the transmitter 1.sub.3 or 2.sub.3, and then when the strength becomes lower than the threshold value, the transmission power is increased, whereby the transmission power is controlled to be increased and decreased stepwisely in accordance with the reception signal strength.
In other words, when the reception signal strength is relatively low, it means that a reception sensitivity is low which may be caused by reasons that the handset unit 2 is positioned far away from the base unit 1 and the like, and in order to practice successful transmission and reception of signals without any malfunction, it is necessary to increase the transmission power. On the contrary, when the reception signal strength is relatively high, it means that the reception sensitivity is high, and in this case, strong transmission power is not required. Due to execute the successful communication between the base and handset units, the transmission power thereat is designed to be controlled in accordance with a judgement made on whether or not a reception signal strength is greater or lower than threshold values stored in the ROM.
However, since the threshold values stored in the ROMs are fixed, there is a risk that a proper control of the transmission power cannot be performed. That is, the detection properties of reception signal strength of IF circuits in ICs may be varied due to production variability and thus some IF circuits used in cordless telephones may supply control circuits with reception signal strengths RSSIs which are different from that of a standard IF circuit, irrespective of the fact that the electric field strengths of signals received by receivers are the same.
Therefore, even if a RSSI value detected at an IF circuit having a detection property different from the standard one is compared with fixed threshold values stored in ROMs and a transmission power control is executed in response to the results of the comparison, a proper control cannot be performed. For instance, it is assumed a case where an IF circuit is used which tends to output the RSSI value of a received signal as a bit greater than that of a standard IF circuit. When a control circuit judges that the RSSI value exceeds the threshold value and then decreases the transmission power, it results in shortage in transmission power, leading to the fact that the transmitted signals cannot be received properly on a reception side.
Referring back to FIG. 1, a key-echo-back function of the conventional cordless telephone is executed as follows. The function is for displaying numbers of dialed keys on the display 2.sub.8 of the handset unit 2. When a dial key on the keyboard 2.sub.7 of the handset unit 2 is depressed,. its key code data is generated by the control circuit 2.sub.5 and then transmitted to the PBX via the transmitter 2.sub.3, duplexer 2.sub.2, and the antenna 2.sub.1, and further via the antenna 1.sub.1, duplexer 1.sub.2, receiver 1.sub.4, control circuit 1.sub.5, and the data transmission/reception circuit 1.sub.6 those are in the base unit 1.
In the PBX, the key code data so received is converted to key echo back data and the converted data is then returned to the control circuit 2.sub.5 of the handset unit 2 via a route reverse to the above route, whereby a number corresponding to the depressed dial key is displayed on the display 2.sub.8 in accordance with the returned key echo back data.
Thus, in the conventional art, since the key code data is sent to the PBX from the cordless telephone and the key echo back data is then sent back from the PBX to the handset unit 2 to display the corresponding dialed number on the display 2.sub.8, this requires a certain period of time before the display. In addition thereto, there may be caused in the base unit 1 a risk that key code data from the handset unit 2 collides with the data from the PBX, resulting in complicated data processing in the base unit 1.
Further, in the conventional cordless telephone as shown in FIG. 1, there is a problem with a ringing tone of the cordless telephone. That is, when a call signal is received at the telephone, it is notified to a user by a ringing tone occurring at the handset unit 2. Although the sound volume of the ringing tone is normally adjustable, the user cannot verify the preset volume at an optional time and thus the user does not know whether the volume has been set at an intended level, until the ringing tone is actually generated by a call signal. This causes the user to get concerned. In addition, since it is difficult to verify the volume of a ringing tone, there is a case where an adjusted sound volume is greatly differs from an intended one, because of unable affirmation.
Moreover, even if the ringing tone volume is set at a minimum volume level, when a user remove the handset 2 unit from a cradle of the base unit 1 and carries it, the notification of reception of a call signal or paging, which is effected by the ringing tone, sometimes becomes very noisy to the user.